The main aim of this study is to develop a comprehensive process model for biomass gasification in a pilot scale bubbling fluidized bed gasifier using the ASPEN PLUS simulator. A drawback in using ASPEN PLUS is the lack of a library model to simulate fluidized bed unit operation. However, it is possible for users to input their own models, using FORTRAN codes nested within the ASPEN PLUS input file, to simulate operation of a fluidized bed. The products of homogeneous reactions are defined by Gibbs equilibrium and reaction rate kinetics are used to determine the products of char gasification. Governing hydrodynamic equations for a bubbling bed and kinetic expressions for the char combustion were adopted from the literature. Different sets of gasification results for the operation conditions (temperature and air equivalence ratio (ER)) obtained from the our pilot-scale gasifier having a capacity of 1 kg/hr of olive kernel as feeding biomass, were used to demonstrate the validation of the model. The simulation results received from the application of the model were compared with the above experimental results and showed good agreement.